The invention relates to a filter composed of cavity resonators, the adaptation of which filter can be adjusted during use. A typical application of the invention is an antenna filter of a base station of some mobile network.
Cavity resonators are generally used in communications networks for making filters, especially when the effect of the signal to be transferred is relatively large. This is due to the fact that losses caused by such resonator filters are small, which means only a slight damping of the effective signal. Additionally their response characteristics are easy to control and adjust even according to strict specifications.
In most filters, both the center frequency and bandwidth of the pass band of the filter is meant to be fixed. In some filters the bandwidth of the pass band of the filter is meant to be fixed, but the center frequency of the pass band can be made adjustable within range of center frequencies. Thus an adjustment possibility for altering the center frequency of the pass band is needed in the filter in addition to the basic cavity filter construction.
FIG. 1 shows an example of such a resonator filter known from publication EP 1604425. The filter 100 has a conductive casing formed by a bottom 101, walls 102 and a lid 105, the space of which casing is divided with conductive partitions 112 into resonator cavities. The figure shows as a cross-section an input resonator 110 and part of a following resonator 120. Each resonator cavity has a inner conductor 111; 121 of the resonator, which inner conductor is connected in a conductive manner by its lower end to the bottom 101 and the upper end of which is in the air, so the resonators are coaxial-type quarter-wave resonators. For adjusting the filter each cavity has a tuning element TE1; TE2. This is a dielectric piece, which is situated directly beneath the lid 105 of the resonator on slide rails, so that it can be moved in the horizontal plane. The moving takes place by means of a control rod RD above the lid, to which rod the tuning element is attached by means of a peg TP passing through an elongated opening SL in the lid. The tuning elements of different resonators are attached to the same control rod. When the control rod is moved, the specific frequencies of all the resonators are altered by the same amount, whereby the pass band of the filter is moved. When each of the tuning elements is completely above the inner conductor the electric lengths of the resonators are at their longest and the pass band of the filter is at its lowest.
Changing the position of the adjustment mechanism of the filter naturally some what affects the adaptation of the filter, i.e. it affects what kind of impedance it is “seen” as from the input wire and correspondingly from the output wire. The change in the adaptation is also manifested from a change in reflection coefficient of the filter: a rise in the reflection coefficient on the pass band of the filter shows a worsening of the adaptation more clearly than a change in the impedance. When the bandwidth of the pass band is relatively small, for example less than a percent of the frequency of the carrier wave of the signal, variation in the level of the reflection coefficient may be insignificantly small. Whenever the pass band is moved over wider range of frequencies, the larger the variation in the level of the reflection coefficient also is. The need for moving the pass band is especially large in a system according to the LTE standard (Long Term Evolution) designed for the 2.6 GHz area. In the filter according to FIG. 1 and in other corresponding known filters the input of the filter is arranged so that the connection to the input resonator and the input impedance are in order in the middle of the adjustment area of the band. This leads to a situation where adaption errors occur in the ends of the adjustment area.
An arrangement for adjusting the input connection of the resonator filter and thus the adaptation of the input side is known from publication U.S. Pat. No. 6,025,764. There is a flexible metal strip in the cavity of the input resonator, which metal strip is attached at its one end to the middle conductor of an input connector. Its free end can be pushed by turning a screw in the side wall of the filter casing and the input connection can thus be changed. The adjustment is thus manual.